1. Field
One or more exemplary embodiments of the present teachings relate to a hyper-branched polymer, an electrode that includes the hyper-branched polymer, an electrolyte membrane that includes the hyper-branched polymer, and a fuel cell employing the electrode and/or the electrolyte membrane.
2. Description of the Related Art
Fuel cells that include a polymer electrolyte membrane operate at relatively low temperatures and can be small in size. Thus, such fuel cells are expected to be used as energy sources in electric vehicles and in distributed generation systems. Perfluorocarbon sulfonic acid-based polymer membranes, such as NAFION membranes, (available from E.I. du Pont de Nemours and Company) are commonly used as polymer electrolyte membranes.
However, such polymer electrolyte membranes should be humidified, in order to sufficiently conduct protons. In addition, to enhance cell system efficiencies, polymer electrolyte membranes should be operated at high temperatures, i.e., at least 100° C. However, the moisture in the polymer electrolyte membrane is evaporated at such temperatures, which reduces the effectiveness thereof.
To address such problems and/or other problems in the related art, non-humidified electrolyte membranes, which may operate at temperatures of at least 100° C., without humidification, have been developed. For example, U.S. Pat. No. 5,525,436 discloses a non-humidified electrolyte membrane made from a polybenzimidazole that is doped with phosphoric acid.
In addition, in cells that operate at low temperatures, such as the cells including a perfluorocarbon sulfonic acid-based polymer membrane, electrodes that include polytetrafluoroethylene (PTFE) as a waterproofing agent have been widely used, to prevent gas diffusion in the electrodes, due to formation of water produced as electricity is generated. For example, Japanese Patent Laid-Open Publication No. 2005-283082 discloses the use of such electrodes.
In addition, phosphoric acid fuel cells, which operate at temperatures of from 150 to 200° C., include a liquid phosphoric acid electrolyte. However, the liquid phosphoric acid interferes with gas diffusion in the electrodes. Therefore, an electrode catalyst layer that includes a polytetrafluoroethylene (PTFE) waterproofing agent, which prevents fine pores in the electrodes from being clogged by the phosphoric acid, has been used.
In addition, in fuel cells including a polybenzimidazole (PBI) electrolyte membrane, which uses a phosphoric acid as a non-humidified electrolyte, to reduce contact between electrodes and the electrolyte membrane, a method of impregnating the electrodes with a liquid phosphoric acid has been used, and a method of increasing a loading amount of metal catalysts has been used. However, such fuel cells do not exhibit improved properties.
In addition, when a phosphoric acid-doped solid polymer electrolyte is used, and air is supplied to the cathode, an activation time thereof is about 1 week, even when an optimized electrode composition is used. Although the performance of the solid polymer electrolyte may be improved, and an aging time may be shortened, as air supplied to the anode is replaced with oxygen, this replacement is undesirable for commercial use. In addition, the polymer electrolyte membrane formed of PBI does not have satisfactory mechanical properties, chemical stability, or the capability of containing a phosphoric acid. Thus, there is still a need for improvement.